In recent years, there has been examined a system in which a camera for monitoring is attached in a car of a railroad system and data obtained by photographing a congestion situation and the like in the car (hereinafter referred to as “image data”) is transferred to display apparatuses set in a motorman's cab, a conductor's compartment, and the like to enable a motorman and a conductor to easily check a situation in the car.
When the camera for monitoring is attached in the car as explained above, crime prevention in the car can be expected. When a crime actually occurs in the car, it is likely that the data photographed by the camera can be used as a proof and an evidence of the crime. However, because the image data is in a digital format, contents of the image data can be easily altered. Reliability of the image data is a problem. As measures against such a problem, there is a technology for performing an electronic signature on photographed image data to thereby guarantee reliability of the data (a proof that the data is not altered) (e.g., Patent Literature 1).